Single-pass, direct-fired generator for an absorption chiller

ABSTRACT

A direct-fired generator for an absorption chiller includes an inner shell, in which combustion occurs, and an outer shell. The inner shell supports a tube bundle through which a first portion an absorption solution is conveyed. Combustion products makes a single pass across the tube bundle within the inner shell. Such construction minimizes the number of potential leak paths and facilitates leak testing of the generator at an intermediate stage of assembly. A flow distributor apportions solution flow to the tube bundle and to a second solution flow path which bypasses the tube bundle but which is likewise heated by the combustion occurring within the inner shell. The two solution flow paths converge after the solution flowing therethrough has been heated by the combustion occurring in the inner shell. A vapor separator disentrains solution in liquid form from vaporized solution before the vapor exits the generator.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a generator for an absorptioncooling system. More particularly, the present invention relates tofluid flow patterns in a direct-fired generator of an absorptionchiller.

[0003] 2. Description of Related Art

[0004] Typical absorption chillers have a refrigerant or working fluidconsisting of at least a two-part solution, such as a solution oflithium bromide and water or ammonia and water. Varying the solution'sconcentration by cyclically vaporizing and reabsorbing of the solution'stwo components allows for the use of a pump or multiple pumps tocirculate the solution through the chiller to create a cooling effect.

[0005] In operation, one or more so-called generators add heat thesolution to raise its absolute pressure and to vaporize one solutionpart. The vaporized part will be referred to hereinbelow as a weak orless concentrated solution and for a solution of lithium bromide andwater, the term “weak solution” refers to pure or nearly pure waterwhich may be found in a liquid or vaporous state downstream of thegenerator. For systems using a solution of ammonia and water, the weaksolution is pure or nearly pure ammonia. The unvaporized portion of thesolution in the generator is referred to as a more concentrated orstrong solution.

[0006] Weak solution flows from the generator of an absorption chillerto a condenser where it is cooled and condensed to liquid form. From thecondenser, the solution flows to and functions as a refrigerant within arelatively lower-pressure evaporator component. The lower pressure foundin the evaporator causes the solution to expand. That expansion furtherlowers the solution's temperature and permits that solution to be usedas a refrigerant to cool still another liquid, most typically water.That cooled liquid is then used as needed, such as to cool rooms orother areas of a building or in an industrial process application.

[0007] After performing its cooling function in the evaporator andvaporizing in the process, the weak solution migrates, in vaporous form,to the absorber component where it is reabsorbed resulting in thecreation of a liquid solution of intermediate concentration. Thatsolution is delivered to the generator component to repeat and gain theeffect of the solution separation process.

[0008] A generator is referred to as being direct-fired if its source ofheat is from direct combustion instead of from steam or waste heatdelivered to the chiller from another process and/or location. Indirect-fired generators, hot combustion gas is typically directed acrossthe exterior of a tube set through which solution of intermediateconcentration flows so as to heat the solution and cause thevaporization of a portion of it.

[0009] The heating of solution in a direct-fired generator ofteninvolves multiple passes of combustion gas across the tube set so as toextract as much heat from the combustion gas as possible. Whileefficient in that regard, multi-pass designs typically add significantlyto the cost and complexity of a generator for the reason that suchdesigns generally have more parts including, but not limited to, a turnbox which redirects the flow of combustion gas from one pass across thetube set to another.

[0010] In so-called single-pass direct-fired generator designs,combustion gas makes only one pass across the tube set. In such designs,an outer shell often surrounds an inner combustion chamber. Combustiongas heats some of the solution as it travels vertically upward throughthe tube set and heats the rest of the solution as it travels upwardbetween the inner and outer shells of the generator.

[0011] In practice, it can be very challenging to manufactureshell-within-shell units. Further, once the shells are assembled andwelded together, it can be very difficult to find and repair any leaksbetween the two that might exist. Even a slight leak can dramaticallyaffect an absorption chiller, not only from a performance standpoint,but from a reliability standpoint. In that regard, the leakage of airinto an absorption chiller can lead to rapid and extensive corrosioninside the unit.

[0012] Other concerns with existing single-pass generator designs exist.For example, rapid upward flow and discharge of solution from thevertical tubes or from between the sides of inner and outer generatorshells in such designs can create a geyser-like effect at the surface ofthe solution pool which is found just above the combustion chamber. Suchdisruption of the solution pool surface tends to cause the vaporoussolution above that pool surface to entrain and carry liquid out of thegenerator and into the system condenser, evaporator, and, eventually,absorber. Any such liquid carryover reduces an absorption chiller'scapacity.

[0013] The need continues to exist for a readily manufacturablesingle-pass direct-fired generator for an absorption chiller wherein thegenerator can be leak tested before final assembly and in whichprovision is made to minimize the carryover of liquid entrained in thevapor that flows out of the generator's interior.

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide an absorptionchiller with a single-pass, direct-fired generator the inner shell ofwhich can be fabricated and completely leak checked before fabricatingthe outer shell.

[0015] Yet another object of the present invention is to apportionliquid solution flow within a direct-fired generator between a firstpath, through the generator's tube bundle, and a second path, whichbypasses the tube bundle, such that most of the heat transfer betweencombustion gas and solution occurs within the tube bundle.

[0016] A further object of the present invention is to provide adirect-fired generator having an inner shell in which less than half ofthe shell volume is taken up by a tube bundle.

[0017] A still further object of the present invention is to provide asingle-pass, direct-fired generator with a vapor separator situated anappreciable distance away from the location of liquid solutiondischarged from the generator's tube bundle.

[0018] Yet another object of the present invention is to provide a vaporseparator for a direct-fired generator having a geometry which inhibitsthe entry of liquid solution into the interior thereof yet out of whichany liquid solution that does enter may readily drain.

[0019] Another object of the present invention is to provide a vaporseparator for a direct-fired generator having flow deflectors thatdirect vapor-entrained liquid droplets away from the generator's vaporoutlet and which assist in creating a vapor flow pattern thatfacilitates liquid disentrainment during the course of vapor flowtherethrough.

[0020] Another object of the present invention is to provide asingle-pass, direct-fired generator whose combustion gas inlet and vaporoutlet are found in a common end plate.

[0021] These and other objects of the present invention are provided bya direct-fired generator for an absorption chiller that includes innerand outer shells having lower, generally U-shaped half-shells welded toinverted, generally U-shaped upper half-shells. The inner shell definesa combustion chamber and supports a tube bundle such that the combustiongas makes a single pass across the tube bundle. The majority of liquidsolution flow within the generator is vertically upward through the tubebundle while a lesser liquid portion flows between the shells. A vaporseparator is disclosed and is disposed within the generator so as tosignificantly limit the carryover of liquid solution out of thegenerator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic diagram of an absorption chiller thatincludes a single-pass, direct-fired generator.

[0023]FIG. 2 shows the generator of FIG. 1 in a cross-sectional viewtaken along line 2-2 of FIG. 1.

[0024]FIG. 3 shows the generator of FIG. 1 in a cross-sectional viewtaken along line 3-3 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] Referring initially to FIG. 1, single-pass, direct-fired, hightemperature generator 10 of the present invention is shown schematicallyto illustrate its relationship with other components of an exemplaryabsorption chiller 12. In addition to generator 10, other majorcomponents of chiller 12 include a condenser 14, an evaporator 16, anabsorber 18 and a low temperature generator 20. It will be appreciatedby those skilled in the art that generator 10 can readily be adapted foruse in absorption chillers having different configurations, fluidcircuiting and component layouts.

[0026] Chiller 12 makes use of a solution 22 which is a solution havingat least one constituent that can be separated from and then reabsorbedinto a second constituent. While chiller 12 will be described withreference to a solution consisting of water and lithium bromide, othersolutions, such as ammonia and water, are also within the scope of theinvention.

[0027] The concentration of solution 22 in the preferred embodiment willvary throughout chiller 12 from weak to strong with the weak solutionbeing pure or nearly pure water. The phase of solution 22 will likewisevary from liquid to vapor/gas depending upon its location within thechiller.

[0028] Solution pumps 24, 25, 26 and 27 circulate solution 22 throughthe various components of chiller 12. The number and type of pumpsemployed by chiller 12 may vary from one chiller design to the next andis not material to the generator of the present invention.

[0029] The purpose of chiller 12 is to cool a liquid, indicated at 28,which passes through heat exchanger 30 of evaporator 16. Liquid 28 canbe water, glycol, a mixture of water and glycol, or another fluid thatis conveyed from chiller 12, once it has been cooled, to wherever it isneeded. For example, liquid 28 can be circulated through a remote heatexchanger (not shown) used in an industrial process or to cool a room orother area within a building. The process by which liquid 28 is chilledwill now be explained in the context of the various components ofchiller 12, starting with direct-fired, high temperature generator 10.

[0030] Generator 10 heats solution 22 which creates within its confinesa weak solution 22 a, consisting primarily of water vapor, and a moreconcentrated solution 22 b, consisting of water in the liquid state witha relatively high concentration of lithium bromide. Concentratedsolution 22 b exits generator 10 through a liquid outlet 32 while weakvaporous solution 22 a passes through a liquid-vapor separator 34 priorto exiting the generator through a vapor outlet 36.

[0031] Following first the flow of weak vaporous solution 22 a, fromvapor outlet 36 of direct-fired generator 10, vaporous solution 22 apasses through a heat exchanger 38, which is disposed within lowtemperature generator 20, in heat exchange contact with solution 22 d.Solution 22 d is of intermediate concentration and is distributed ontoheat exchanger 38 from reservoir 40 within the low temperature generatoras will further be described.

[0032] The heat from solution 22 a vaporizes solution 22 d within lowtemperature generator 20. This results in the creation of a weakvaporous solution 22 e within the upper portion thereof and a moreconcentrated liquid solution 22 b at the bottom thereof. Weak vaporoussolution 22 e migrates through vapor separator 42 into condenser 14.

[0033] A heat exchanger 44 exists within condenser 14 through whichwater flows. That water is often water which has been cooled by aconventional cooling tower. Heat exchange between the water flowingthrough heat exchanger 44 and vapor 22 e within the condenser coolsvapor 22 e and causes it to condense. The condensate collects at thebottom of condenser 14 and mixes with weak solution 22 a, which isreceived from heat exchanger 38 in the low temperature generator, toform a pool of relatively cool weak liquid solution 22 c within thecondenser.

[0034] Weak solution 22 c is conveyed by line 43 to the relatively lowerpressure evaporator 16. As this weak solution is fed into the relativelylower pressure evaporator it expands and its temperature drops further.As a result, a pool of weak liquid solution 22 f of relatively lowtemperature is created within the evaporator. That solution iscirculated upward within evaporator 16 by pump 24, is fed into reservoir46 and is directed thereoutof onto heat exchanger 30. The flow of lowtemperature solution 22 f onto heat exchanger 30 cools liquid 28 whichit is, once again, the purpose of chiller 12 to cool.

[0035] As a result of the heat exchange process within the evaporator,solution 22 f absorbs heat from liquid 28, vaporizes and migratesthrough a vapor separator 48 into absorber 18. Pump 26 circulatessolution 22 d of intermediate concentration to distributor 49 withinabsorber 18 which, in turn, distributes that solution onto heatexchanger 50. The distributed solution flows downward through heatexchanger 50 and through an atmosphere of vapor 22 g within theabsorber. As a result of this process, solution 22 d absorbs vapor 22 gand then collects at the bottom of the absorber.

[0036] Pump 25 then pumps solution 22 d from the absorber to replenishthe supply of more concentrated solution in low temperature generator 20while pump 27 pumps solution from low temperature generator 20 todirect-fired generator 10 to replenish the supply of more concentratedsolution there. As will be noted, as solution is conveyed to lowtemperature generator 10 and to direct-fired generator 20, it ispreheated within heat exchangers 52 and 54 by the recovery of whatotherwise would be waste heat from liquid solution that flows from thegenerators.

[0037] Referring primarily now to FIGS. 2 and 3, the structure ofdirect-fired, high temperature generator 10 includes an inner shell 56surrounded by an outer shell 58. Inner shell 56 includes a generallyU-shaped lower inner shell section 56 a and an inverted, generallyU-shaped upper inner shell section 56 b. Each of sections 56 a and 56 bis preferably a unitary piece which is continuously formed from end toend. That is, the U-shape is preferably not created by a series ofindividual panels welded or otherwise fastened together though theycould be. Sections 56 a and 56 b are welded along two substantiallyparallel lap joints 60. To avoid or minimize corrosion at joints 60,lower shell section 56 a fits inside upper section 56 b which preventsthe creation of a pocket or ledge on which liquid solution 22 b mightotherwise collect. Shell 56, once assembled, comprises a two-piece firetube/tube sheet assembly of simple design and manufacture having openrectangular ends.

[0038] A tube bundle 62, which includes a group of vertical heattransfer tubes through which solution is conveyed upward withingenerator 10, extends across the interior of inner shell 56. The upperand lower tube ends are welded to upper and lower shell sections 56 band 56 a respectively. The welds are made on the solution side of thetube/shell interface to avoid corrosion of the weld by exposure tocombustion products. End plates 64 and 66 are then welded to oppositeends of the inner shell.

[0039] End plate 64 includes vapor outlet 36, as earlier noted, and acombustion inlet 68 to which burner 69 is attached and through which aburning combustion fluid 70 is introduced into the interior of shell 56,generally upstream of the tube bundle in an area referred to as the firetube portion of the shell. End plate 66 includes a combustion outlet 72through which combustion products exit the shell's interior after makinga single pass therethrough.

[0040] Once welded together, inner shell sections 56 a and 56 b, tubebundle 62, and end plates 64 and 66 can be readily leak checked as aunit by attaching leak check covers to the combustion inlet and outlet.If a leak is discovered, all welded joints are readily accessible forrepair.

[0041] Similar in construction to inner shell 56, outer shell 58includes a generally U-shaped lower outer shell section 58 a and aninverted, generally U-shaped upper outer shell section 58 b. Like thesections of inner shell 56, each of sections 58 a and 58 b is preferablya continuously formed piece, as opposed to being created by a series ofindividual panels, and are welded/joined along two substantiallyparallel lap joints 74. To avoid or minimize corrosion due to liquidcollection and stagnation at the joint location, upper shell section 58b fits inside lower section 58 a.

[0042] Before welding sections 58 a and 58 b together, vapor separatorassembly 34 which, in the preferred embodiment, includes a V-shapedtrough 34 a, inner deflectors 34 b and outer deflectors 34 c, isassembled into upper outer shell section 58 b. Outer shell sections 58 aand 58 b are then welded along lap joints 74 and end plates 64 and 66are welded thereto.

[0043] In operation, solution 22 d, of intermediate concentration,enters solution inlet chamber 80, defined generally at the bottom ofgenerator 10 and within channel 85, after passing through inlet 82. Aliquid inlet flow distributor 76 can be created by providing lower shellsection 58 a with apertures 84 and enclosing those apertures withinchannel 85 which is welded to the underside of lower shell section 58 a.Channel 85 can, but need not, be considered to be an integral part oflower section 58 a and distributor 76 could be configured so as to bedisposed internal of inlet chamber 80.

[0044] Apertures 84 can vary in size and/or spacing to apportion andrestrict, in a controlled manner, the flow of solution into the interiorof outer shell 58. Those of apertures 84 which are located under tubebundle 62 are preferably larger and/or their spacing is closer so as tocause more solution to flow upward and into tube bundle 62 than flowsupward between the walls of shells 56 and 58. For that reason, most ofthe heat transfer between combustion fluid 70 and solution 22 withingenerator 10 is at the location of the tube bundle. Regardless of whichflow path the solution follows, it makes its way into a outlet chamber86 which is located within shell 58, above inner shell 56.

[0045] The vaporization of solution that occurs within generator 10 as aresult of its being heated creates a more concentrated solution 22 b inthe upper region of the generator. That solution readily mixes with andassimilates the incoming, less concentrated solution 22 d which itselfbecomes more concentrated in its flow upward through the generator.

[0046] As hot combustion products travels from inlet 68 to outlet 72within inner shell 56, they make a single pass across the exterior oftube bundle 62 thereby heating the solution flowing inside the tubes.However, a significant amount of heat also transfers through the wallsof inner shell 56 and heats the portion of the solution that flowsupward between the walls of the inner and outer shells.

[0047] In the preferred embodiment, tube bundle 62 takes up less thanhalf the interior volume of inner shell 56 which leaves ample space foropen-flame combustion upstream of the tube bundle without having toresort to a special, more costly burner that produces a compact flamefor purposes of avoiding direct and detrimental flame contact with theexterior of the tubes of the tube set. Generally speaking, most or allof tube bundle 62 is downstream of midpoint 71 of the length ofgenerator 10 in the preferred embodiment.

[0048] Vapor 22 a travels out of outlet chamber 86 within generator 10,into and through separator 34 which helps to disentrain any liquid fromthe vapor 22 a prior to its exit from the generator interior. Outerdeflectors 34 c operate to initially deflect liquid solution that mayspew upward from between the walls of shells 56 and 58 away from trough34 a and from vapor outlet 36 which is found therein. One end 88 oftrough 34 a is blocked off while an opposite end 90 is open to vaporoutlet 36. Inlet slits 92 along upper edges of trough 34 a allow vapor22 a to enter the trough's interior.

[0049] Once inside trough 34 a, the geometry of the trough and itsinterior deflectors 34 b cause the vapor to swirl generally along thelength of the trough. That swirling motion slings remaining liquiddroplets 22 h within vapor 22 a against an interior surface 94 of thetrough. Those droplets accumulate along the bottom of the trough untilsufficient in amount to drain out of the trough's open end 90. The netresult of the separator configuration is that vapor 22 a exits throughvapor outlet 36 only after traveling through a tortuous path and aftermuch of its previously entrained liquid is removed.

[0050] Although the generator of the present invention is described withreference to a preferred embodiment, it will be appreciated by thoseskilled in the art that other variations are well within the scope ofthe invention. For example, generator 10 can be used in single-stage ormulti-stage absorption chillers. Also, the various components of chiller12 can be rearranged in a variety configurations. The shells ofgenerator 10, auxiliary generator 20, condenser 14, absorber 18, andevaporator 16 can be individual shells interconnected by piping orvarious combinations of shells which share a common wall. Therefore, thescope of the invention is to be determined only with reference to theclaims, which follow.

We claim:
 1. A generator for an absorption chiller that uses a burningcombustion fluid to heat a solution, comprising: a tube bundle, saidtube bundle conveying said solution through a first solution flow pathin a heat exchange relationship with said combustion fluid; an innershell, said inner shell being vertically traversed by said tube bundleand defining a combustion inlet and a combustion outlet, said burningcombustion fluid and the combustion products thereof flowing in a singlepass from said combustion inlet, across said tube bundle, to and out ofsaid combustion outlet; an outer shell, said outer shell defining aliquid solution outlet and a vapor solution outlet, said outer shellcooperating with said inner shell to define a solution inlet chamber, asolution outlet chamber and a second solution flow path in parallel withsaid first solution flow path, a first portion of said solution flowingthrough said liquid solution inlet, through said solution inlet chamber,through said first solution flow path and into said solution outletchamber and a second portion of said solution flowing through saidliquid solution inlet, through said solution inlet chamber, through saidsecond solution path and into said solution outlet chamber; and adistributor in flow communication with said solution inlet chamber, saiddistributor causing more solution flow through said first solution flowpath than through said second solution flow path.
 2. The absorptiongenerator of claim 1 wherein said combustion fluid path through saidinner shell has a midpoint generally halfway between said combustionfluid inlet and said combustion fluid outlet, at least a majority of thetubes of said tube bundle being downstream of said midpoint with respectto the direction of flow of said combustion fluid.
 3. The absorptiongenerator of claim 2 wherein all of the tubes of said tube bundle aredownstream of said midpoint.
 4. The absorption generator of claim 1wherein said flow distributor is attached to said outer shell and is inflow communication with said inlet chamber through a plurality ofapertures defined in said outer shell.
 5. The absorption generator ofclaim 4 wherein said apertures are spaced and/or sized to deliver moresolution to said first solution flow path than to said second solutionflow path.
 6. The absorption generator of claim 1 further comprising aliquid-vapor separator disposed within said solution outlet chamber,said separator having an inlet, the interior of said separator being inflow communication with said separator inlet and said vapor outlet. 7.The absorption generator of claim 6 wherein said liquid-vapor separatoris configured to impart a swirling motion to solution received throughsaid separator inlet.
 8. The absorption generator according to claim 6wherein said liquid-vapor separator includes a trough having a blockedend and an open end, said trough being configured to receive solutionfrom said solution outlet chamber through said separator inlet, toconvey a portion of said received solution, in a vaporous state, throughsaid open end of said separator to said vapor solution outlet, to conveya portion of said received solution in a liquid state along an interiorsurface of said trough to said open end, and to drain said liquidportion of said received solution out of said open end of said troughback into said solution outlet chamber.
 9. The absorption generator ofclaim 8 wherein, said liquid-vapor separator includes an inner deflectordisposed within said trough adjacent to said trough inlet, said innerdeflector directing solution entering said trough said separator inlettoward said interior surface of said trough.
 10. The absorptiongenerator of claim 9 wherein said liquid-vapor separator includes anouter deflector disposed in said solution outlet chamber, said outerdeflector being positioned to deflect solution entering said solutionoutlet chamber from said second solution path away from said inlet tosaid liquid-vapor separator.
 11. The absorption generator of claim 1further comprising a first end plate, in which said combustion fluidinlet and said vapor solution outlet are defined, and a second endplate, in which said combustion fluid outlet is defined, said first andsecond end plates each being attached to said inner shell and said outershell in a leak-tight manner.
 12. The absorption generator of claim 11further comprising a liquid-vapor separator, said separator beingdisposed adjacent said vapor solution outlet defined by said first endplate.
 13. The absorption generator of claim 1 wherein said outer shellincludes a generally U-shaped lower outer shell section and an invertedgenerally U-shaped upper outer shell section.
 14. The absorptiongenerator of claim 13 wherein said lower outer shell section has twoedges joined to said upper outer shell section to create twosubstantially parallel leak-tight joints, said two edges of said lowerouter shell section being attached to the exterior of said upper outershell section.
 15. The absorption generator of claim 13 wherein one endof the tubes of said tube bundle are welded to said lower inner shellsection and the other end of the tubes of said tube bundle are welded tosaid upper inner shell section.
 16. The absorption generator of claim 1wherein said inner shell includes a generally U-shaped lower inner shellsection and an inverted generally U-shaped upper inner shell section,said upper inner shell section having two edges joined to said lowerinner shell section to create two substantially parallel leak-tightjoints.
 17. The absorption generator of claim 16 wherein said two edgesof said upper inner shell are joined to the exterior of said lower innershell section.
 18. A generator for an absorption chiller that uses aburning combustion fluid to heat a solution, comprising: a tube bundleadapted to convey said solution through a first solution flow path inheat exchange relationship with said combustion fluid; an inner shell inwhich said tube bundle is disposed, said inner shell defining acombustion fluid inlet, a combustion fluid outlet and a combustion fluidpath through which said combustion fluid makes only one pass across saidtube bundle in traveling from said combustion fluid inlet to saidcombustion fluid outlet, said inner shell having a generally U-shapedlower inner shell section and an inverted generally U-shaped upper innershell section; and an outer shell defining a liquid solution inlet, aliquid solution outlet and a vapor solution outlet, said outer shellbeing disposed around said inner shell to define therebetween a solutioninlet chamber, a solution outlet chamber and a second solution flow pathin parallel with said first solution flow path, a first portion of saidsolution flowing through said liquid solution inlet, through saidsolution inlet chamber, through said first solution flow path and intosaid solution outlet chamber and a second portion of said solutionflowing through said liquid solution inlet, through said solution inletchamber, through said second solution flow path and into said solutionoutlet chamber.
 19. The absorption generator of claim 18 furthercomprising a flow distributor that directs more solution flow into saidfirst solution flow path than into said second solution flow path. 20.The absorption generator according to claim 19 further comprising aliquid-vapor separator disposed in said solution outlet chamber adjacentsaid vapor solution outlet, said separator having an inlet into whichsaid solution flows.
 21. The absorption generator according to claim 20wherein said combustion fluid inlet and said vapor solution outlet areon one end of said generator and said combustion fluid outlet is at asecond end of said generator, said combustion fluid entering said innershell, combusting adjacent said combustion fluid inlet in a flameproducing process and proceeding therefrom down said combustion fluidpath, in the form of products of combustion, toward said combustionfluid outlet, said tube bundle being disposed in said inner shelldownstream of said flame.
 22. The absorption generator of claim 18wherein said combustion fluid path has a midpoint halfway between saidcombustion fluid inlet and said combustion fluid outlet, most of thetubes of said tube bundle being downstream of said midpoint with respectto the direction of flow of said combustion fluid.
 23. The absorptiongenerator of claim 22 wherein all of the tubes of said tube bundle aredownstream of said midpoint.
 24. The absorption generator of claim 18further comprising a liquid-vapor separator disposed within saidsolution outlet chamber, said liquid-vapor separator comprising atrough, said trough having a blocked end and an open end and defining atrough inlet located between said blocked end and said open end, saidtrough receiving solution from said outlet chamber through said troughinlet, discharging a portion of said solution, in a vaporous state,through said open end and into said vapor solution outlet, conveying aportion of said solution in a liquid state along an interior surface ofsaid trough and draining said solution portion in said liquid state fromsaid interior surface back into said solution outlet chamber.
 25. Theabsorption generator of claim 24 wherein said liquid-vapor separatorfurther comprises an inner deflector disposed within said trough,adjacent to said trough inlet, said inner deflector directing solutionfrom said trough inlet toward said interior surface of said trough. 26.The absorption generator of claim 25 further comprising an outerdeflector disposed in said solution outlet chamber between said troughand said outer shell, said outer deflector being adapted to deflect theportion of said solution that enters said solution outlet chamber fromsaid second solution flow path away from said trough inlet.
 27. Theabsorption generator of claim 18 wherein said outer shell has agenerally U-shaped lower outer shell section and an inverted generallyU-shaped upper outer shell section.
 28. The absorption generator ofclaim 27 wherein said upper inner shell section has two edges joined tosaid lower inner shell section to create two substantially parallelleak-tight joints and wherein said lower outer shell section has twoedges joined to said upper outer shell section to create twosubstantially parallel leak-tight joints.
 29. The absorption generatorof claim 28 further comprising a first end plate in which saidcombustion fluid inlet and said vapor solution outlet are defined and asecond end plate in which said combustion fluid outlet is defined, saidfirst and said second endplates cooperating with said upper inner shellsection, said lower inner shell section, said upper outer shell sectionand said lower outer shell section to create said inner and said outershells.
 30. The absorption generator of claim 28 wherein said two edgesof said lower inner shell section are joined to an interior surfaceinside of said upper inner shell section and said two edges of saidlower outer shell section are joined to an exterior surface of saidupper outer shell section.
 31. The absorption generator of claim 18wherein said tube bundle extends through said lower inner shell sectionto and through said upper inner shell section and wherein the tubes ofsaid tube bundle are welded to said inner shell on the exterior of saidinner shell so that said tube welds are not directly exposed to saidcombustion fluid.
 32. A method of heating a solution with a combustionfluid in the generator of an absorption chiller, comprising the stepsof: conveying a first portion of said solution through a first solutionflow path defined by a tube bundle; conveying a second portion of saidsolution through a second solution flow path, said second flow pathparalleling said first flow path, at least a portion of said secondsolution flow path being upstream of said tube bundle with respect tothe direction of flow of said combustion fluid through said generator;apportioning solution into said first solution flow path and into saidsecond solution flow path so that the amount of solution flowing throughsaid first solution flow path is larger than the amount of solutionflowing through said second solution flow path; conveying saidcombustion fluid across said tube bundle only one time, so as to heatsaid first portion of said solution flowing through said tube bundle;heating said second portion of said solution in said second flow pathwith the same combustion fluid which is conveyed across said tubebundle; and conveying both said first portion and said second portion ofsaid solution to a location generally above said tube bundle.
 33. Themethod according to claim 32 comprising the further step of conveyingsaid solution from said location above said tube bundle into aliquid-vapor separator disposed proximate said location above said tubebundle.
 34. The method according to claim 33 comprising the furthersteps of burning said combustion fluid, said burning step creating aflame; and, disposing said tube bundle in said generator so as not to bein direct contact with said flame.
 35. The method of claim 34 comprisingthe further steps of disentraining solution which is in liquid form fromsolution which is in vapor form within said liquid-vapor separator;flowing solution in vapor form from which solution in liquid form hasbeen disentrained out of said generator; and returning disentrainedliquid solution to said location generally above said tube bundle. 36.The method of claim 35 comprising the further step of shielding theinlet of said liquid-vapor separator from solution conveyed to saidlocation through said second solution flow path.